Get cursor count in Mongodb with aggregation framework - mongodb

Question

I'm working with mongo and node. I have a collection with a large number of records an unknown number of which are duplicates. I'm trying to remove dups following Remove duplicate records from mongodb 4.0 and https://docs.mongodb.com/manual/aggregation/ .
So far I have:
db.hayes.aggregate([
... {"$group" : {_id:"$PropertyId", count:{$sum:1}}}
... ]
... );
{ "_id" : "R135418", "count" : 10 }
{ "_id" : "R47410", "count" : 17 }
{ "_id" : "R130794", "count" : 10 }
{ "_id" : "R92923", "count" : 18 }
{ "_id" : "R107811", "count" : 11 }
{ "_id" : "R91389", "count" : 15 }
{ "_id" : "R22047", "count" : 12 }
{ "_id" : "R103664", "count" : 10 }
{ "_id" : "R121349", "count" : 12 }
{ "_id" : "R143168", "count" : 8 }
{ "_id" : "R85918", "count" : 13 }
{ "_id" : "R41641", "count" : 13 }
{ "_id" : "R160910", "count" : 11 }
{ "_id" : "R48919", "count" : 11 }
{ "_id" : "M119387", "count" : 10 }
{ "_id" : "R161734", "count" : 12 }
{ "_id" : "R41259", "count" : 13 }
{ "_id" : "R156538", "count" : 7 }
{ "_id" : "R60868", "count" : 10 }
to get the number of groups I tried in the mongo shell:
> const cursor = db.hayes.aggregate([{"$group" :
{_id:"$PropertyId", count:{$sum:1}}} ]);
> cursor.count()
uncaught exception: TypeError: cursor.count is not a function :
#(shell):1:1
Apparently this works with the db.cllection.find statement. How do I do this with the aggregate framework?

Answer 1

Add the following stage after the group stage to see the groups count:
{$count:"Total"}

Answer 2

Count method on the cursor changes the query being sent from find to count. This only works if you are sending a find query to begin with, i.e., not when you are aggregating.
See https://docs.mongodb.com/manual/reference/method/cursor.count/#cursor.count which includes guidance for how to count when aggregating.